This invention generally relates to apparatus and methods for inspection of welds and more particularly relates to apparatus and methods for radiographic inspection of welds, which welds may be seal welds of the kind typically found on nuclear power reactor control rod drive mechanisms.
Although devices and methods for radiographing welds are known in the prior art, it has been observed that prior art devices and methods have certain operational problems associated with them which make these devices and methods unsuitable for radiographing welds of the kind typically found on nuclear power reactor control rod drive mechanisms. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear power reactor and its associated control rod drive mechanism.
In this regard, a nuclear power reactor is a device for producing heat by the controlled fission of fissionable material contained in fuel rods. A plurality of the fuel rods are bundled together by a plurality of spaced-apart grids, each grid having open fuel rod cells for receiving each fuel rod therethrough. Each grid also has open guide tube cells for receiving respective ones of a plurality of control rod guide tubes. Each control rod guide tube is in turn capable of slidably receiving a movable control rod for controlling the fission process in the nuclear reactor. The fuel rods, control rod guide tubes and grids define a fuel assembly, a plurality of which fuel assemblies are grouped in a sealed reactor pressure vessel to define a nuclear reactor core. Pressurized liquid moderator coolant (i.e., borated demineralized water) is caused to circulate through the pressure vessel and over the fuel rods in the reactor core for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the fuel rods.
Clusters of the previously mentioned movable control rods are each connected to a control rod drive mechanism (CRDM) which is attached to the top of the reactor pressure vessel. The CRDMs are capable of selectively vertically positioning the control rods within the fuel assemblies to adjust the core reactivity so that the fission process is suitably controlled thereby. For this purpose, the control rods that are connected to the CRDMs can be withdrawn from or inserted into the core by the CRDMs at speeds consistent with the reactivity changes required for controlling the fission process. Thus, during normal reactor operation, the CRDMs serve to axially move or to hold in position the control rods that have been withdrawn from the reactor core for sustaining the fission process.
The moving internal components of each CRDM, which is capable of moving the control rod cluster connected thereto, are contained in a pressure housing attached to the top of a reactor vessel head adaptor, which is in turn attached to the top of the reactor pressure vessel. The pressure housing is threadably connected to the vessel head adaptor and seal welded thereat for sealing the joint defined by the threaded connection. The pressurized reactor coolant previously mentioned fills the pressure vessel head adaptor and the pressure housing and immerses all moving components of the CRDM to serve as lubricant for the CRDM components. Thus, the vessel head adaptor, which is attached to the top of the reactor pressure vessel (i.e., attached to the reactor pressure vessel closure head), forms part of the pressure boundary of the reactor pressure vessel. Consequently, in the typical nuclear reactor, the pressure housing and the vessel head adaptor are designed for full system pressure of approximately 2500 pounds per square inch absolute (psia).
Applicant has observed that, due to weld degradation, small pinhole leaks of borated coolant have occurred at the location of the seal welds sealing the threaded joint defined by the pressure housing and the reactor vessel head adaptor. Such a CRDM pressure housing leak ultimately could lead to undesirable reactivity anomalies in the reactor core because the inventory of the borated moderator coolant used to assist in controlling the fission process is momentarily reduced by the leak. In addition, because the pressure housing of the CRDM forms a portion of the reactor pressure boundary, any seal weld leaks may affect the pressure and thus the temperature of the moderator coolant. Variation in the coolant temperature also undesirably affects core reactivity. Such leaks might also cause corrosion of the low alloy base metal comprising the pressure housing of the CRDM and may thereby compromise the structural integrity of the CRDM. Although the precise cause of the weld degradation is unknown, it is believed that the factors contributing to such leaks may be faulty welding or contaminants in the weld. Moreover, the process of welding the consumable filler inserts used to form the seal welds during the welding process may result in an insufficient amount of filler metal being deposited. Furthermore, the weld anomalies could also be related to improper weld temperature control or insufficient gas purging at the locus of the weld during the welding process. In addition, the weld degradation may be related to thermal cycling which the welds might undergo during reactor heat-up and cool-down.
Degraded seal welds are repaired by overlaying the seal weld with additional filler material, the repaired weldment then being subjected to grinding to produce a smooth weld. However, it has been observed by applicant that such a repaired weld may also leak if there is insufficient fusion of the filler and base metals or if the weldment is excessively ground and then subjected to the relatively high system pressure of normal reactor operation.
It is therefore important to inspect the seal welds that seal the threaded joint between the pressure housing and the vessel head adaptor to detect any weld anomalies that might lead to leakage of the coolant. It is prudent to perform this inspection even after any weld anomalies have been repaired in order to confirm the sufficiency of the repair. In this regard, it is current practice in the art to visually inspect the exterior surfaces of seal welds on approximately ten percent of the CRDMs during hydrostatic tests. In the case of visual inspection, the American Society of Mechanical Engineers (ASME) Code requires that there must be sufficient access to the weld to allow the eye to be within 24 inches of the exterior surface of the weld and at an angle not less than 30 degrees to the surface inspected. Mirrors can be used to improve the angle of vision. The ASME Code also allows use of a remote camera for visual examination of the seal welds; but, the remote camera systems must have a resolution capability as good as obtainable by direct visual inspection. However, applicant has observed that visual inspection techniques may not obtain the required level of reliability because such techniques are inherently dependent on the visual acuity of the maintenance personnel performing the visual inspection. In addition, in many cases visual examination of exposed surfaces cannot detect locations of impending leaks due to subsurface weld anomalies.
Ultrasonic examination may be used to detect subsurface weld anomalies such as subsurface cracks, local thinning, or other anomalies. Moreover, dye penetrant inspections, magnetic particle testing, and eddy current inspection may also be used to detect subsurface weld anomalies. However, it has been observed by applicant that use of these devices and methods is time consuming and generally lack the required sensitivity and are therefore unsuitable for reliably detecting relatively small pinhole anomalies in the seal weld.
Prior art methods have been used to detect anomalies both on the surface and in the subsurface of welds. A method of radiographic inspection of a joint formed between an end-cap and a tubular can which forms part of a nuclear fuel element is disclosed in British Patent 1,195,620 titled "Method of Inspection of Welds" published Jun. 17, 1970 in the name of Pierre Soulat. This patent discloses that to inspect the welded joint between a tube and its end cap, the tube is continuously rotated about its axis at a peripheral linear speed equal to the speed at-which a film, detecting the X-rays after passage through the tube, is displaced tangentially to the tube, the X-ray beam being directed perpendicularly to the film. Although the Soulat patent may disclose a method of radiographic inspection of a welded joint, the Soulat method apparently requires that the workpiece be rotated in order to perform the inspection. Thus, the Soulat patent does not appear to disclose an apparatus and method for suitably radiographing CRDM seal welds because the CRDM is stationary.
X-ray equipment for mechanized testing of the integrity of welds along the length of the welds is disclosed in U.S. Pat. No. 4,078,180 titled "X-Ray Inspection of Welds" issued Mar. 7, 1978 in the name of Donald T. Green. This patent discloses means for mechanically traversing an X-ray source along one side of the weld and a grainless fluorescent screen along the other side, with the screen coupled to an image-isocon video camera. According to this patent, photographic film or magnetic tape can be substituted for the camera. More specifically, this patent discloses an X-ray tube inserted in a pipe and projecting a conical X-ray beam through the weld in the wall of the pipe. On the outside of the pipe facing the X-ray tube is a fluoroscopic screen placed close to the outside of the pipe to obtain a picture of the flaw in the weld. Although the Green patent may disclose X-ray equipment for testing the integrity of welds, this patent does not appear to disclose an apparatus and method for suitably radiographing relatively small pinhole-sized anomalies in CRDM seal welds.
Therefore, what is needed is an apparatus and method for radiographing a weld, which weld may be a seal weld of the kind typically found on nuclear power reactor control rod drive mechanisms.